1. Field of the Invention
The invention is related to the field of magnetoresistance (MR) elements and, in particular, to using a chemically disordered magnetic material to form a free layer and/or a pinned layer of an MR read element.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. A suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride at a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves an actuator arm that is connected to the suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
As the read element passes over the bits and bit transitions recorded on tracks of the magnetic disk, the magnetic fields for the bits and bit transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element, and then measuring the change in bias voltage across the read element. The resulting read back signal is used to recover the data encoded on the track of the magnetic disk.
The most common type of read elements are magnetoresistance (MR) read elements. One type of MR read element is a Giant MR (GMR) read element. GMR read elements having two layers of ferromagnetic material (e.g., CoFe) separated by a nonmagnetic spacer layer (e.g., Cu) are generally referred to as spin valve (SV) elements. A simple-pinned SV read element generally includes an antiferromagnetic (AFM) pinning layer (e.g., PtMn), a ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic spacer layer (e.g., Cu), and a ferromagnetic free layer (e.g., CoFe). The ferromagnetic pinned layer has its magnetization fixed by exchange coupling with the AFM pinning layer. The AFM pinning layer generally fixes the magnetic moment of the ferromagnetic pinned layer substantially perpendicular to the ABS of the recording head. Substantially perpendicular means closer to perpendicular than to parallel. The magnetization of the ferromagnetic free layer is not fixed and is free to rotate in response to an external magnetic field from the magnetic disk.
Another type of SV read element is an antiparallel (AP) pinned SV read element. The AP-pinned SV read element differs from the simple pinned SV read element in that an AP-pinned structure has multiple thin film layers forming the pinned layer structure instead of a single pinned layer. The pinned layer structure includes a first ferromagnetic pinned layer (e.g., CoFe), a nonmagnetic AP-coupling layer (e.g., Ru), and a second ferromagnetic reference layer (e.g., CoFe). The first ferromagnetic pinned layer has a magnetization oriented in a first direction substantially perpendicular to the ABS by exchange coupling with the AFM pinning layer. The second ferromagnetic reference layer is antiparallel coupled with the first ferromagnetic pinned layer across the AP coupling layer. Accordingly, the magnetization of the second ferromagnetic reference layer is oriented in a second direction that is generally antiparallel to the direction of the magnetization of the first ferromagnetic pinned layer.
It is desirable in a GMR read element to have high spin-polarization in the free layer and the pinned layer of a simple spin-valve or the free layer and the reference layer of an AP-pinned spin-valve to achieve high GMR amplitudes. The high GMR amplitudes produce a cleaner read back signal that is read from the magnetic disk. One problem with using CoFe or a similar material for the pinned layer and the free layer is that the spin-polarization may not be as high as desired. One solution to this problem is to replace the CoFe with another material having a higher spin-polarization.
One class of materials exhibiting 100% spin-polarization is half-metallic full Heusler alloys. Full Heusler alloys are defined as chemically ordered alloys that exhibit the chemical formula X2YZ and crystallize in the L21 structure. The L21 structure comprises a simple cubic lattice of X atoms. The Y and Z atoms are located in the alternating body-centered cubic sites of the simple cubic lattice of X atoms thus forming two interpenetrating face-centered cubic lattices. FIG. 1 illustrates an exemplary structure of an ordered Heusler alloy. Examples of half-metallic full Heusler alloys are Co2MnAl, Co2MnSi, Co2MnGe, and Co2FeSi. The full Heusler alloys yield high GMR amplitudes when used for the free layer and/or pinned layer of a simple spin-valve or the free layer and/or the reference layer of an AP-pinned spin-valve. One problem however with full Heusler alloys is that their high spin-polarization leads to high spin-torque noise because the current-density threshold for spin-torque is generally lower for higher spin-polarization alloys. The high spin-torque noise distorts the read back signal making this alloy undesirable for use as a free layer or a pinned layer. Thus, it is desirable to use a different type of material for the pinned layer and/or free layer of a GMR read element.